For such suspensions, it is known that damping devices which dissipate the mechanical energy as heat by shearing of oil driven by a piston through throttled passages, does not incorporate all the characteristics desired, and in particular lacks insufficient reliability.
In fact, the most common damping devices, of the telescopic piston type actuated by a rod in a cylinder filled with oil, do not generally permit suitable damping of oscillating movements of small amplitude or vibrations generated by the wheels, since they are easily initiated at rest and then necessitate long paths with oscillations of large amplitude to return to the normal operating stage. However the piston sucks in first, in each direction of movement, the air dissolved in the oil; on its return, this creates a vacuum causing the cavitation phenomenon manifested by particularly unpleasant hard and violent jerks.
To overcome these drawbacks, shock-absorbers are known in which a slight permanent overpressure of the liquid completely filling the working chambers is maintained, by means of a compensating chamber where the reserve liquid is itself kept under pressure by spring or pneumatic devices.
However, in the field of railway suspensions, it is known that the oscillations to be damped are in the majority of cases vibrations of small amplitude with, from time to time, one or several distinctly bigger oscillations.
This causes a sealing problem around the rod of the piston in its passage through the jacket of the apparatus. In fact, if seals are used with considerable friction on the rod, the small oscillating movements are transmitted through the seals to the whole of the body. It is possible to reduce the friction a little so as to permit between the rod and the seal small amplitude movements which are then damped by the working of the fluid in the chambers; however in this case the very numerous oscillating movements result in rapid wear of the seals, and consequently leakages of oil which, in their turn, will result in a reduction in the permanent pressure in the apparatus and the appearance of cavitation phenomena.
This drawback is particularly troublesome in the railway field, since the mileage travelled by a vehicle between two major maintenance operations is generally considerable, so that effective active life of the shock absorbers is often well below the time between two successive adjustments.
Sealing devices formed by a diaphragm seal which grips the piston rod are also known, but the elasticity and the motion of this type of seal are distinctly insufficient to avoid its slippage with respect to the piston rod, as well as premature wear.